1. Field of the Invention
The present invention relates to surface texture measuring apparatuses, and more particularly to measurement of an object (workpiece) having a steep unevenness.
2. Description of the Related Art
Surface texture measuring apparatuses are commonly known that scan a surface of a workpiece with a detector having a stylus to measure unevenness (or roughness) of the surface.
In such a surface texture measuring apparatus, the stylus is moved in a fixed direction (direction of the X axis), and displacement of the stylus in the vertical direction (direction of the Z axis) caused by unevenness of the workpiece is converted to electric signals, which are sampled by a scale signal or a fixed time signal, so that the unevenness is displayed on a display device or printed by a printing device as a function of the traveled distance (in the X axis direction).
FIG. 4 shows the values detected in accordance with a conventional sampling method. In FIG. 4, the horizontal axis is the X axis indicating the traveled distance of the detector in the X axis direction (position in the X axis direction), and the vertical axis indicates displacement in the Z axis direction, i.e. the amount of unevenness. A dot in the figure denotes the value detected at a sampling time. The values are sampled at an interval of a fixed distance L in the X axis direction, and output as detected values (x, z).
On the other hand, in a common commercially available surface texture measuring apparatus, the values are detected by sampling a detection signal at a fixed time interval, rather than by sampling at an interval of a fixed distance in the X axis direction, because the detector moves at a fixed speed. However, precisely speaking, the moving speed of the stylus is not constant, and accurate calibration of the moving speed is difficult. Therefore, a high-precision surface texture measuring apparatus is provided with a scale disposed along the X axis to accurately measure the position of the stylus (or detector) with respect to the workpiece for sampling.
When the surface of the workpiece is relatively smooth with only a slight unevenness, data can be sampled at substantially equal intervals along the traveling direction (X axis) of the detector either by an X axis scale signal or by a fixed time signal. However, when the workpiece has a surface with a steep unevenness, the sampling intervals along the surface of the workpiece are not always constant, resulting in a problem that proper data on unevenness of the surface of the workpiece cannot be obtained.
For example, even by the method of sampling data at intervals of a fixed distance L as shown in FIG. 4, when a steep slope is present at the surface of the workpiece, data is not sampled until a position L2 after a position L1, and therefore unevenness data is lost between the positions L1 and L2. Thus, the shape of the surface of the workpiece cannot be accurately ascertained.
In addition, when such a sudden change occurs in the Z axis direction, the stylus may not accurately follow the surface of the workpiece due to limitation of following frequency response of the stylus if the detecting stylus is driven at a fixed speed.
An object of the present invention is to provide a surface texture measuring apparatus that ensures detection of unevenness of the surface of a workpiece even when the surface has a steep unevenness.
In order to achieve the above object, the present invention, according to one aspect of the invention, provides a surface texture measuring apparatus for causing relative movement of a stylus in a first axis direction along a workpiece to detect displacement of the stylus in a second axis direction perpendicular to the first axis direction resulting from the surface texture of the workpiece, comprising means for detecting the amount of displacement in the second axis direction, and means for changing a detection interval in the first axis direction in accordance with the amount of displacement. Thus, the detection interval (interval between L1 and L2 in FIG. 4) in the first axis direction is not fixed but changed in accordance with the amount of displacement in the second axis direction, making it possible to deal with a steep unevenness. Preferably, the detection interval is decreased as the amount of displacement increases.
The means for changing a detection interval preferably outputs the position in the first axis direction and the displacement in the second axis direction obtained when the amount of displacement reaches or exceeds a threshold. Consequently, detection timing is determined by the amount of displacement in the second axis direction when the surface of the workpiece has a steep unevenness.
When the amount of displacement is smaller than the threshold, the means for changing a detection interval preferably outputs the position in the first axis direction and the displacement in the second axis direction obtained when the amount of movement in the first axis direction reaches a predetermined value. As a result, detection timing is determined by the amount of movement in the first axis direction when the surface of the workpiece does not have a steep unevenness. In summary, the detection timing is determined by the amount of displacement in the second axis direction when the surface of the workpiece has a steep unevenness, while it is determined by the amount of displacement in the first axis direction when a steep unevenness is not present, making it possible to deal with either a smooth or a rough surface of the workpiece. When the amount of displacement is smaller than the threshold, it is also possible to output the position in the first axis direction and the displacement in the second axis direction at the time of reaching a predetermined sampling time.
Alternatively, the means for changing a detection interval preferably outputs the position in the first axis direction and the displacement in the second axis direction on the earlier occasion of the amount of displacement reaching or exceeding the threshold, and the amount of movement in the first axis direction reaching a predetermined value. When the surface of the workpiece has only a slight unevenness, the amount of movement in the first axis direction first reaches the predetermined value to determine the detection timing. On the other hand, when the surface has a steep unevenness, the amount of displacement in the second axis direction first reaches the threshold to determine the detection timing.
According to another aspect of the invention, the present invention provides a surface texture measuring apparatus for moving a stylus in a first axis direction along a workpiece to detect displacement of the stylus in a second axis direction perpendicular to the first axis direction resulting from the surface texture of the workpiece, comprising means for detecting the amount of displacement in the second axis direction, and means for changing a moving speed of the stylus in the first axis direction in accordance with the amount of displacement. The moving speed in the first axis direction is not fixed but is varied in accordance with the amount of displacement in the second axis direction, to thereby allow the stylus to accurately follow the surface of the workpiece. Preferably, the moving speed is decreased with an increase in the amount of displacement. Alternatively, the moving speed can be decreased when the amount of displacement reaches or exceeds a threshold.
According to a still another aspect, the present invention provides a surface texture measuring apparatus for moving a stylus in a first axis direction along a workpiece to detect displacement of the stylus in a second axis direction perpendicular to the first axis direction resulting from the surface texture of the workpiece, comprising means for detecting the amount of displacement in the second axis direction, means for changing a detection interval in the first axis direction in accordance with the amount of displacement, and means for changing a moving speed of the stylus in the first axis direction in accordance with the amount of displacement. The detection interval in the first axis direction (interval between the positions L1 and L2 in FIG. 4) is changed in accordance with the amount of displacement in the second axis direction, and the relative speed is also changed, thereby improving response performance of the stylus and detection resolution, and achieving highly precise measurement.